/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func trimBST(root *TreeNode, low int, high int) *TreeNode {
    if root == nil {return nil}
    if root.Val < low {
        // 寻找符合右区间的节点
        return trimBST(root.Right, low , high)
    }
    if root.Val > high {
        // 寻找符合左区间的节点
        return trimBST(root.Left, low, high)
    }
    root.Left = trimBST(root.Left, low, high)
    root.Right = trimBST(root.Right, low, high)
    return root
}